Switch valves have been used for decades in various industrial applications to divert an upstream fluid to one of a plurality of downstream lines extending from the switch valve. Conventional switch valves have generally utilized one of two basic designs: (1) a plug valve element design, or (2) a ball valve element with bellows design.
Switch valves utilizing plug element have not proven to be highly reliable, particularly when the valves are used in severe service automated applications. When the plug element is lifted and rotated to operate the valve, particulate matter frequently becomes trapped between the plug element and the seat, which may damage the seat and thus increase the likelihood of leakage. A typical ported plug valve cannot handle the full flow of the pipe to which it is connected, and is difficult to automate. Accordingly, ball element switch valves are generally preferred over plug element switch valves when used in severe service automated applications. A common severe service application for a switch valve is a coker operation, wherein hydrocarbon is diverted by the switch valve from an upstream source to one of a plurality of downstream lines. Those skilled in the art appreciate that this type of service for a valve requires high reliability since coke formed from the hydrocarbon tends to foul moveable components within the valve. Other types of severe service applications will be apparent to those skilled in the art.
A significant problem with prior art switch valves of the ball variety with bellows relates to the complexity of the valve. Such valves commonly have as many as sixteen internal parts, including a ball assembly and a seat, sleeve, bellows, insert gasket, and insert associated with each of the plurality of outlets. Each bellows seals between the valve body and a respective seat, and biases the seat for sealing engagement with the ball. The large number of parts increases the likelihood of valve failure due to a failure of one or more of these parts. This valve design utilizes a separate bellows loading a seat adjacent to each of the outlets to balance and centralize the ball in the center of the valve body. The bellows assembly conventionally includes a weld between the bellows and the seat, and this assembly depends upon a balanced purge to prevent the bellows from being damaged by system over pressure. If the valve loses purge pressure, the bellows may be distorted and the valve permanently damaged.
Other problems with prior art switch valves of the ball variety relate to the high costs to maintain the valve. To disassemble a conventional switch valve with a ball and multiple bellows, the weld from the sleeve to the bellows must be cut and a new sleeve welded in place. Since various severe service applications commonly use chrome-moly materials for these components, this welding operation is difficult and requires stress relieving to minimize the likelihood of cracking. Moreover, the use of an extremely thin material for the bellows assembly is required, and care must be taken in selecting a material with high tensile and yield properties. The bellows assembly materials are thus expensive, and the thin materials could limit valve testing and full service rating in actual service.
Still another problem with conventional switch valves utilizing a ball element and multiple bellows is that the bellows design allows for the accumulation of material passing through the valve to build up and adversely affect the valve operation. In view of the high number of areas where buildup may occur, a large number of purge ports are provided in conventional ball-type switch valves. A good deal of time and expense is thus required to minimize buildup of solid matter in these areas.
When the ball port of a prior art switch valve moves during rotation of the stem to divert fluid from one outlet to another outlet, the cavity around the ball is exposed to the process stream. In addition, the cavity purge exhausts with the full bore of the ball as the ball rotates, thereby allowing unrestricted flow and high consumption of the steam purge. This loss of steam is costly and the ingress of process material could lead to valve lockup, both of which are significant problems for this type of design.
Those skilled in the art recognize that switch valves having at least two and often three outlet ports present unique problems with respect to their design and operation compared to conventional shut-off or control valves having a single inlet port and a single outlet port. The disadvantages of the prior art are overcome by the present invention, and an improved switch valve utilizing a ball element is hereinafter disclosed. The switch valve of this invention is highly reliable, and is particularly well suited for severe service applications.